When a digital signal is transmitted through a transmission line, at a signal receiving side, waveform distortion such as overshoot or undershoot, in other words, ringing, occurs when a portion of signal energy is reflected at the timing when a signal level changes. Thus, several proposals have been made for suppressing waveform distortion. For example, Patent Literature 1 discloses that, when the voltage level of the transmission line is changed between a high level and a low level, the impedance is set to be consistent in a predetermined period in which the communication is not affected to suppress ringing.
However, in Patent Literature 1, the period for making the impedance to be consistent is fixed. Accordingly, in a situation where an actual communication speed is lower than an expected speed, there is less advantageous effect in suppressing ringing. In addition, in a situation where the actual communication speed is faster than the expected speed, a communication signal cannot be normally received. In order to avoid such situation, it is required to design a product corresponding to an individual speed. However, an increase in number of components or cost is expected.